JP H08(1996)-230441A (Application No. 1995-37164) shows a battery system arranged to perform a charging operation of charging an in-vehicle battery installed in an electric vehicle and a driving operation of driving at least one of various loads such as an air conditioner for adjusting an inside temperature of the vehicle during the charging operation, by supplying power from an external power source to the battery and the load. In order not to hamper the charge of the battery, this battery system is basically arranged to supply power to the air conditioner by using a surplus power excluding power required by the battery from the available power which the external power source can supply.